Jacobitz S, Bishop P E
Agricultural Research Service, U.S. Department of Agriculture, Raleigh, North Carolina.
J Bacteriol. 1992 Jun;174(12):3884-8. doi: 10.1128/jb.174.12.3884-3888.1992.
Under diazotrophic conditions in the absence of molybdenum and in the presence of vanadium, Azotobacter vinelandii reduces N2 to NH4+ by using nitrogenase-2, a V-containing enzyme complex encoded by vnfH (the gene for dinitrogenase reductase-2), and vnfDGK (the genes for dinitrogenase-2 subunits). Accumulation of the vnfHorfFd and vnfDGK transcripts occurred under Mo-deficient conditions in the presence and absence of V; however, in the case of vnfDGK, the protein products only accumulated in the presence of V. This suggests that V is required for translation of the vnfDGK transcripts. In addition, expression of vnfH-lacZ and vnfD-lacZ transcriptional fusions was only partially repressed in the presence of NH4+. Transcripts hybridizing with vnfH (1.4 and 1.0 kb), vnfDG (3.4 and 1.8 kb), and vnfK (3.4 kb) were detected in RNA extracted from wild-type cells cultured with NH4+ in the presence or absence of V. However, nitrogenase-2 subunits were not detected in extracts of cells derepressed for nitrogenase-2 in the presence of NH4+. These results indicate that this nitrogen source acts at the posttranscriptional level as well as at the transcriptional level. vnf transcripts were not detected in the presence of Mo (with or without NH4+).
在缺乏钼且存在钒的固氮条件下，棕色固氮菌利用固氮酶-2将N2还原为NH4+，固氮酶-2是一种含钒的酶复合物，由vnfH（双氮酶还原酶-2的基因）和vnfDGK（双氮酶-2亚基的基因）编码。在缺乏钼的条件下，无论有无钒，vnfHorfFd和vnfDGK转录本都会积累；然而，就vnfDGK而言，蛋白质产物仅在有钒的情况下积累。这表明钒是vnfDGK转录本翻译所必需的。此外，在存在NH4+的情况下，vnfH-lacZ和vnfD-lacZ转录融合体的表达仅被部分抑制。在用NH4+培养的野生型细胞提取的RNA中，检测到与vnfH（1.4和1.0 kb）、vnfDG（3.4和1.8 kb）和vnfK（3.4 kb）杂交的转录本。然而，在存在NH4+的情况下，对于固氮酶-2去阻遏的细胞提取物中未检测到固氮酶-2亚基。这些结果表明，这种氮源在转录后水平以及转录水平上都起作用。在存在钼（无论有无NH4+）的情况下未检测到vnf转录本。
